if you prefer not to use any phoenix downs, you should be able to kill him easily, he only has 9999 hp, it isn't really that hard. or have you been neglecting that dreadful fact that in almost every rpg you have to take some time off the sensational storyline to level?

Vivi's bio should be able to inflict 1100-2000 at that time, and all of your character should have body temp by now. Eiko should have already mastered phoenix if you need to revive someone. If you want to steal some items just be sensible and have the anti-bad status abilities equiped on.

This goes for everyone who is playing this game. let your characters learn every ability they can!